Most secondary power systems for aircraft normally make use of the following different power sources: (1) Engine compressor (bleed air) power, (2) pneumatic power, (3) hydraulic power, (4) electrical power, and (5) mechanical power. These multiple power systems are individually complex and require a significant amount of design development and testing. They are also extremely costly to produce and require a significant amount of maintenance and ground logistic support.
Military aircraft pose special problems not found in commercial aircraft to the extent that they cannot usually rely on the availability of ground support equipment for powering hydraulic, pneumatic, electronic and air conditioning systems as well as engine start functions. Such is the case where military aircraft are located in dispersed and "bare site" areas where the necessary ground support equipment may not be on hand.
A typical prior art combined starting and auxiliary power system can be found in U.S. Pat. No. 2,582,848 "Aircraft Power Plant and Cabin Pressurizing System," by N. C. Price. Price discloses a system wherein an electric motor is used to drive a compressor which supplies pressurized air to charge a storage tank. To start the engine, air from the tank is ducted to a turbine which is mechanically coupled to the main engine by a drive shaft. Once the engine is started, however, bleed air from the engine is used to power the environmental control system of the aircraft. This system has the disadvantage of requiring bleed air from the engine to run the environmental control system which greatly affects the performance parameters of the engine such as thrust and fuel consumption. Furthermore, the storage tank adds weight to the aircraft.
Another system of interest can be found in U.S. Pat. No. 2,777,301 "All-Purpose Power and Air Conditioning System," by J. Kuhn. Kuhn discloses the use of a conventional auxiliary power unit comprising a turbine, compressor and combustor and further incorporates a generator mounted on the drive shaft between the compressor and the turbine. The output of the compressor is used to supply high pressure air to a pneumatic starter. Separate turbine and compressor assemblies are required to produce conditioned air for the cabin. Here the disadvantage is the weight penalty due to the need for a separate turbine and compressor for air conditioning.
Another example of using compressed air from a gas turbine to start a jet engine can be found in U.S. Pat. No. 3,965,673 "Apparatus for Starting Aircraft Engines and For Operating Auxiliary On-Board Power Generating Equipment," by Helmut Friedrich. Other patents of interest include U.S. Pat. No. 3,834,161 "Dual Mode Auxiliary Power Unit," by Richard E. Quigley, Jr, and Buryl L. McFadden, Jr.; U.S. Pat. No. 4,149,371 "Air Supply Control System," by Wilbur A. Spraker and Kenneth F. Kaiser; U.S. Pat. No. 3,279,169 "Arrangement for Starting and Adjusting the Operation of a Power Generator Incorporating a Gas Turbine," by Gaston Bayard; U.S. Pat. No. 4,312,179 "Gas Turbine Power Plant with Air Reservoir and Method of Operation," by Paul Zaugg; and U.S. Pat. No. 4,196,773 "Heating and Air Cooling System Employing a Gas Turbine," by Paul R. Trumpler.
Therefore, a primary object of the present invention is to provide a multi role primary/auxiliary power system for an aircraft that also provides an engine start function for at least one aircraft engine.
It is another object of the present invention to provide a multi role primary/auxiliary power system for an aircraft that provides the aircraft with self-sufficiency when external ground support equipment is unavailable.
A further object of the subject invention is to utilize engine shaft power to drive the electro and turbo-machinery so as to furnish mechanical, electical and air conditioning power.
A still further object of the invention is to utilize internal and external electric power to provide the electric and air conditioning loads in an aircraft.